The present invention relates to a method for balancing a battery pack comprising a plurality of battery cells connected in series, wherein each battery cell is associated with a resistor which is connected in parallel with said battery cell and wherein each of said resistors is coupled in series with a controllable switch which is connected to a control unit. The method comprises selectively closing and opening said switch so as to initiate discharging of the corresponding battery cell, thereby balancing said battery cell in relation to other cells of said battery pack.
The invention also relates to a system for balancing a battery pack comprising a plurality of battery cells connected in series, wherein each battery cell is associated with a resistor which is connected in parallel with said battery cell wherein each of said resistors is coupled in series with a controllable switch which is connected to a control unit, said control unit being configured to selectively close and open said switch to initiate discharging of the corresponding battery cell, thereby balancing said battery cell in relation to other cells of said battery pack.
The invention can be applied in vehicles, such as cars, trucks, buses and construction equipment. Although the invention will be described below with respect to an application in the form of a bus, the invention is not restricted to this particular type of vehicle, but may be used in other vehicles.
In the field of vehicles, there is a steady increase in research and development related to propulsion of vehicles with alternative power sources, i.e. power sources being used as alternatives to conventional internal combustion engines. In particular, electrically operated vehicles have emerged as a promising alternative.
According to today's technology, a vehicle can be operated by means of an electric machine solely, or by means of an arrangement comprising both an electric machine and an internal combustion engine. The latter alternative is often referred to as a hybrid vehicle (HEV), and can for example be utilized in a manner in which an internal combustion engine is used for operating the vehicle while driving outside urban areas whereas the electric machine can be used in urban areas or in environments in which there is a need to limit the discharge of harmful pollutants such as carbon monoxide and oxides of nitrogen.
The technology involved in electrically operated vehicles is closely related to the development of electrical energy storage systems, such as battery-related technology for vehicles. Today's electrical energy storage systems for vehicles may comprise a battery pack with a plurality of rechargeable battery cells which, together with control circuits, form a system which is configured for providing electric power to an electric machine in a vehicle.
A vehicle which is operated by means of an internal combustion engine and an electric machine supplied with power from a rechargeable electrical energy storage system is sometimes referred to as a plug-in hybrid electric vehicle (PHEV). A plug-in hybrid electric vehicle normally uses an energy storage system with rechargeable battery cells which can be charged through a connection to an external electric power supply. During charging, a high amount of energy is fed into the energy storage system in a relatively short time in order to optimize the vehicle's range of driving. For this reason, the actual charging of the energy storage system is suitably implemented through a process in which a control unit on the vehicle requests a charging process to be carried out by means of an external electric power supply. This is carried out after the energy storage system and the external power supply have been electrically connected by means of suitable connector elements.
In the field of automotive technology, an energy storage system normally comprises a battery pack with a large number of battery cells. Using a plug-in hybrid vehicle as an example, a battery pack may for example be of the lithium-ion type. In the event that a 600 V lithium-ion battery pack is used, approximately 200 battery cells connected in series will then be needed to achieve a desired voltage in order to operate the vehicle. The available range for driving the vehicle depends on certain parameters such as the state of charge (SoC) of the battery pack. The state of charge is an important parameter to use in order to prevent batteries from being operated during under- or over-charging situations, and to manage the energy in electric vehicles. The state of charge needs to be estimated since no direct measurement is available for this parameter.
Furthermore, it is known that batteries degrade over time, and there is a need for a diagnosis of any decrease in performance estimated by means of battery parameters, such as for example the cell terminal voltage, the cell capacity and the ohmic resistance of each cell. Such estimation could be carried out by means of a sensor arrangement configured for measuring one or more parameters which can consequently be used as an indication of the state of operation of the battery pack.
Generally, there is a desire to obtain optimum properties of a battery pack. It is previously known that requirements for optimum battery properties can be reached by means of a process referred to as battery cell equalization. The reason is that the voltages of different battery cells in a battery pack will differ between cells during the course of time. This lack of balance between cells may result in degraded battery properties and needs to be corrected.
Today, there exist several different methods for cell equalization. One such known method is to discharge a selected battery cell, which is found to have a cell voltage or state of charge (SoC) which differs considerably from the remaining battery cells, through a resistor which is coupled in parallel with the battery cell.
The patent document US 2013/278218 discloses a circuit for balancing a number of battery cells which comprises a plurality of resistors which are arranged in parallel with the battery cells and a plurality of switches configured in series with the resistors. A control circuit is configured to activate the switch based on the detected voltage of the battery cells. The operating life of the battery is also taken into account.
A disadvantage with this method is that it leads to a high amount of leak currents
from the battery cells due to activation of the battery cell switches for which equalization is to be carried out. This leads to a degradation of the total efficiency of the battery pack.
Consequently, although there exists known systems for equalizing the cells of a battery pack, there is a problem in the form of a need to minimize leak currents from the battery pack. In this manner, the general performance of the battery pack could be optimized.
Consequently, it is desirable to provide an improved method and system for battery cell balancing which solve the problems associated with prior solutions and by means of which a lower level of leak currents and power losses from the battery pack will be obtained.
According to an aspect of the present invention, a method is provided for balancing a battery pack comprising a plurality of battery cells connected in series, wherein each battery cell is associated with a resistor which is connected in parallel with said battery cell and wherein each of said resistors is coupled in series with a controllable switch which is connected to a control unit. The method comprises: selectively closing and opening said switch so as to initiate discharging of the corresponding battery cell; thereby balancing said battery cell in relation to other cells of said battery pack. Furthermore, the method comprises the steps of: a) defining a control variable indicating, for each switch, an open or closed condition;
b) determining a cost based at least on the current losses of the battery pack resulting from said switch being controlled according to said control variable; c) repeating steps a) and b) a predefined number of times; d) selecting a control variable which results in said cost being minimized; and e) initiating said balancing based on the selected control variable.
By means of the invention, certain advantages are obtained. Primarily, it can be mentioned that an improved cell balancing process is obtained in which the power losses due to leak currents in the individual battery cells can be minimized. This leads to improved performance of the battery pack. The invention also allows a continuous cell balancing process with an increased useful energy capacity and increased battery system lifetime as compared with known systems. The process can furthermore be obtained by means of inexpensive hardware and software. For example, the above-mentioned switches only need to handle small currents and can therefore be small and inexpensive.
According to an embodiment, the method comprises determining said cost by executing and minimizing a cost function which defines a relationship between said current losses resulting from a given control variable and the state of balance of the battery pack. According to an embodiment, the method furthermore comprises a step of calculating, for each battery cell of said battery pack, a sum of the current losses and a value which is a function of a the state of balance for the entire battery pack; and selecting the control variable based on a cell which said sum is the lowest of all the cells of said battery pack. In this manner, providing a low loss cell balancing process can be achieved.
According to further embodiments, the above-mentioned control variable can be defined in several ways (referring to step a) above). According to a preferred alternative, it is defined in a manner by using only those battery cells which present a relatively high state of charge, for example exceeding a predetermined threshold value. This limits the load on the hardware and software during the step of selecting a suitable control variable. Evaluating all possible combinations of leaking cells is not computational feasible. For example, for just 8 battery cells there are 256 possible combinations. The approach is consequently to simplify the problem by only leaking the cells with the highest SoC. The exact number of leaking cells is found by minimizing the previously described cost function.
According to another aspect of the present invention, a system is provided for balancing a battery pack comprising a plurality of battery cells connected in series, wherein each battery cell is associated with a resistor which is connected in parallel with said battery cell and wherein each of said resistors is coupled in series with a controllable switch which is connected to a control unit, said control unit being configured to selectively close and open said switch to initiate discharging of the corresponding battery cell, thereby balancing said battery cell in relation to other cells of said battery pack. Furthermore, the control unit is configured for a) defining a control variable indicating, for each switch, an open or closed condition; b) determining a cost based at least on the current losses of the battery pack resulting from said switch being controlled according to said control variable; c) repeating steps a) and b) a predefined number of times; d) selecting a control variable which results in said cost being minimized; and for e) initiating said balancing based on the selected control variable.
Further advantages and advantageous features of the invention are disclosed in the following description.